The invention relates to a heterostructure bipolar transistor power amplifier module, and more particularly to a power amplifier module with a dynamic voltage supply.
As the wireless industry for handheld phones matures, it is insufficient for a power amplifier module (PAM) to meet a specification at a low cost, and with a small footprint while accepting whatever power efficiency at power back-off occurs once the efficiency is optimized at maximum power. The power efficiency drops dramatically with lower output power due to the impedance mismatch between the constant low impedance output match and the rising impedance of the output stage. This is of particular importance with Code Division Multiple Access (CDMA) technology, because in CDMA the probability of the output power is the greatest between 12 to 18 dB power back-off from a nominal maximum and falls to a minimum at either a maximum or minimum power limit.
A simple yet popular approach for improving power efficiency in the power back-off high probable area is to change the ‘mode’ of operation of the amplifier. By moving the ‘mode of operation’ to more of a class B amplifier from a class A amplifier, the efficiency improves. This is achieved by lowering the quiescent current through the RF stages using a simple switch in the DC bias circuitry. The amplifier is switched between states or modes at a 8 to 12 dB back-off, and the efficiency improvement is typically a few percent.
Another approach is to use a DC—DC converter for the main power supply for the power amplifier. Reducing the collector supply voltage(s) when the PAM output requirement is at a lower power level can result in a much higher efficiency improvement, by 100 to 300% (depending on the power level), because the output transistor impedance stays relatively constant with lower power as the supply voltage is also lowered. However, for most HBT amplifiers, this approach only works in the collector voltage range of 4 Volts down to about 1.5 Volts. At best, the output stage collector voltage (Vcc2 for a two stage amplifier) may be reduced to below 1.5 Volts. This limits the useful dynamic output power range to about 10 dB.
FIG. 1 shows the probability distribution function of the transmission power of handheld phone for CDMA for urban areas and a data mode. It can be seen that PAM works only 4% of the time in its enhanced efficiency state whereas the PAM works about 17% of the time in the data mode.